A major goal of the project is the elucidation of factors regulating blood flow in the renal medulla. Among the questions we wish to answer are: does the blood flow vary with the diuretic state of the animal, and if it does, is control exerted in the afferent arteriole, efferent arteriole, or in the vasa recta? Another question is whether there is autoregulation of inner medullary blood flow, because early studies suggested there was not. To study these questions, we propose to implement a video based modification of the dual slit method for estimating erythrocyte velocity. This method permits continuous estimation of velocity and radius in several vasa recta, and these variables will be monitored in states of antidiuresis, water diuresis, saline diuresis, and with alterations of arterial blood pressure. Another major goal requires the measurement of interstitial driving forces on capillary fluid movement. We will perfuse individual capillaries with solutions of varying protein concentration. We will measure capillary hydrostatic pressure and net fluid movement and construct a curve of fluid movement versus the sum of capillary hydrostatic and oncotic pressures. The sum at which fluid movement is zero will correspond to the net interstitial pressure. This method will be implemented first on rat mesentery, to test its feasibility. It will then be applied to the renal cortex, to test hypotheses of proximal tubule function, and finally to the renal medulla, to test hypotheses to tubular reabsorption there.